This invention relates to optical fiber communication systems and, in particular, to a new type of waveguide router for multiplexing and demultiplexing wavelength division multiplexed (WDM) signal channels in such systems.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals, and amplifiers for amplifying the signals along the transmission path. It also includes a receiver for detecting the signals and for demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Optical routers are important components in WDM fiber communication systems. Such devices perform the function of multiplexing separately modulated different wavelength channels for transmission on a single transmission fiber. They also separate the transmitted signal into the plurality of constituent channels so that they can be separately demodulated at a receiver or be added, or dropped or rerouted at intermediate points along the transmission path.
Different optical router designs utilize different dispersive elements to separate or recombine different wavelength channels. One promising router design utilizes a diffraction grating comprising a plurality of different length waveguides. See, for example, U.S. Pat. No. 5,002,350 issued to C. Dragone on Mar. 26, 1991, which is incorporated herein by reference. A particularly advantageous design achieving low levels of crosstalk and insertion loss comprises a waveguide grating connecting a pair of star couplers. See U.S. Pat. No. 5,136,671 issued to C. Dragone on Aug. 4, 1992, which is incorporated herein by reference. In this device a multiplexed signal entering a first star coupler is dispersed by the waveguide grating and the different wavelength channels are presented at the second star coupler on different angularly spaced apart output ports. Conversely, different channels applied to the corresponding ports of the second star coupler are presented as a single multiplexed signal at the first star coupler.
A second type of optical router is based upon a reflective Echelle grating. In essence, this router is a waveguiding slab device comprising an input port directed toward a reflective grating formed along a concave arc. The grating disperses a signal from the input port and, because of its concave arc arrangement, reflects the separated channels back in the general direction of the input port where they can be taken out by angularly separated output ports. Conversely, signals inputted into the corresponding xe2x80x9coutputxe2x80x9d ports will be combined and the multiplexed signal can be taken out the xe2x80x9cinputxe2x80x9d port. The preferred arcs, referred to as Rowland circles or generalized Rowland circles, permit ease of input and ease of fabrication. Such devices are described in detail in R. Merz et al., xe2x80x9cOn the theory of planar spectrographsxe2x80x9d, Journal of Lightwave Technology, Vol. 10, No. 12, pp. 2017-22 (December 1992); and J. He et al., xe2x80x9cMonolithic integrated wavelength demultiplexer based on a waveguide Rowland circle grating in InGaAsP/InPxe2x80x9d, Journal of Lightwave Technology, Vol. 16, No. 4, pp. 1-7 (April 1998). All three references are incorporated herein by reference.
A difficulty with these conventional devices is that they provide relatively little freedom in design. In the Dragone router, the dispersive power is provided solely by differences in waveguide length, and in the reflective Echelle grating devices, the shape of the device defines the dispersive power. Moreover, it is difficult to miniaturize high quality reflective surfaces needed for the grating.
Accordingly there is a need for a WDM optical communication system with a new optical router providing enhanced freedom of design without the use of miniature reflective surfaces.
In accordance with the invention, an optical router for an optical communications system comprises a pair of transmissive Echelle gratings having their grating surfaces coupled by a waveguide grating. The arrangement provides for substantial design freedom in that the dispersive parameters include the shapes of the first and second Echelle gratings as well as the path length difference among the waveguides. Moreover the device eliminates any need for reflective surfaces in the Echelle gratings.